The invention relates to a device for the optical coupling of a solid-state laser to an optical fiber, an anamorphic lens system being arranged between the output face of the solid-state laser and the input face of the optical fiber and converting differing apertures in the main sections of the solid-state laser into essentially identical apertures at the input face of the optical fiber, and to a method for its manufacture.
For efficient coupling of laser light into optical fibers, particularly into single-mode optical fibers, it is necessary to achieve minimum-loss coupling of the optical fiber to the laser. This can be effected by lenses applied to the fiber end. In the case of solid-state lasers, it is also desirable to minimize to the greatest extent possible the loss of light caused by coupling the laser to the fiber. For this purpose, it has become known, for instance, from R. Zengerle, H. J. Bruckner, H. W. P. Koops, H.- J. Olzhausen, G. Zesch, A. Kohl and A. Menschig in xe2x80x9cFabrication of Optical Beamwidth Transformers for Guided Waves on InP Using Wedge-Shaped Tapersxe2x80x9d, J. Vac. Sci. Technol. B9(6), (1991) 3459, to use lithography to define and integrate a made-to-order coupling taper on the laser directly in the laser material. Such tapers, however, are only able to adapt the wave field to the phase in one section. Owing to the epitactic growth and the plane lithography used for the structuring, no adaptation is possible in the other section lying normal to the waveguide.
In addition, a device as set forth by the species defined in the Main Claim is known from U. Griebner, R. Grunwald, H. Schxc3x6nnagel, OSA Proceedings on Advanced Solid-State Lasers, 1995, Vol. 24, 253, which, however, requires considerable expenditure for adjustments. Devices having in each case several lenses to be adjusted and fixed in position are also described in U.S. Pat. No. 5,140,608, British Patent Application No. GB 2 220 501 A, German Patent Application No. DE 39 19 484 A, European Patent Application No. EP 0 484 276 A, Dutch Patent Application No. NL 8600844 A and European Patent Application No. EP 0 706 070 A.
The object of the present invention is to propose a device for coupling a solid-state laser to an optical fiber, which permits substantially no-loss coupling and an adaptation of the wave fields in the direction of both main sections, and which can be manufactured with the necessary accuracy.
This objective is achieved according to the invention, in that the lens system is composed of one anamorphic lens applied on the output face and a further lens applied on the input face.
The device of the present invention has the advantage that in each case, the lenses can be applied on the output face and input face, respectively, with very high accuracy. An adjustment is then only necessary between the laser axis and the axis of the optical fiber. Both the anamorphic lens and the further lens can be advantageously manufactured using well-known processes.
It may be that an anamorphic lense on the input face is not ruled out in the device of the present invention, however, the further lens is preferably spherical. Moreover, this specific embodiment of the invention permits two designs, namely that the anamorphic lens applied on the output face of the laser is an elliptical lens, or is formed of two crossed, cylindrical partial lenses. The elliptical lens or at least one of the component lenses may be designed as a Fresnel lens.
The advantageous specific embodiment of the device according to the invention can further be designed in such a way that a spacer layer, corresponding to the magnification necessary in the direction of the larger aperture, is provided between the anamorphic lens and the output face.
Besides a real imaging of the output pupil of the laser, the device of the present invention makes it possible for the output pupils of the laser to form a virtual circular source image. This measure makes it possible to select a small distance between the lenses or between the output and input faces, accompanied by relatively great focal lengths of the lenses. The great focal lengths of the lenses, in turn, are more favorable for production using microtechnique methods.
Various well-known methods are suitable in principle, such as the definition of the cylinder lenses on the input face of the optical fiber with the aid of high-resolution electron-beam lithography and subsequent fabrication by reactive dry etching. Such a method is described, for instance, in xe2x80x9cHigh Resolution Electron Beam Lithography for Fabricating Visible Semiconductor Lasers with Curved Mirrors and Integrated Hologramsxe2x80x9d by P. Unger, V. Boegli, P. Buchmann and R. Germann, Microelectronic Eng. 23, (1994) 461 and in xe2x80x9cFabrication of curved mirrors for visible semiconductor lasers using electron-beam lithography and chemical assisted ion-beam etchingxe2x80x9d by P. Unger, V. Boegli, P. Buchmann and R. Germann, J. Vac. Sci. Technol., B. 11(6) (1993) 2514-2518. Machining from resist lenses applied on the input or output face is also possible.
However, a particularly advantageous method for producing the device of the present invention is to define the lenses and/or spacer layers and/or antireflection layers with the aid of a dry-resist technique and to fabricate them with the aid of additive lithography, in particular, electron-beam lithography. This method represents a substantial improvement, especially if it is supported by computer programming.
The dry-resist technique, described, for example, in the German Patent 195 31 859.5 A1, makes available a method in which, using vapor deposition in high vacuum, the laser or the fiber end is covered with a defined layer thickness of a polymer which is sensitive to electrons. This polymer is cross-linked by the electron beam during the exposure to form a polymer which is rich in silicon oxide and whose refractive index is well-matched to that of the fiber material (n=1.48), see H. W. P. Koops, S. Babin, M. Weber, G. Dahm, A. Holopkin, M. Lyakhov, xe2x80x9cEvaluation of Dry Resist Viny-T8 and Its Application to Optical Microlensesxe2x80x9d, Microelectronic Engineering 30 (1996), 539. A mirror composed of silicon oxide is applied by vapor deposition to the laser end, the refractive index of the lens material likewise being well-matched to that of the mirror. Thus, the insertion loss of these lenses manufactured from dry resist can theoretically be disregarded.
Using additive lithography with electron-beam-induced deposition, the lenses are directly constructed from precursor molecules, adsorbed from the vapor phase, by electron-beam polymerization and cross linking, using computer control of the beam and the dose, without any need for a previous coating or subsequent development of the structure. Such a method is described by H. W. P. Koops, R. Weiel, D. P. Kern and T. H. Baum in xe2x80x9cHigh Resolution Electron Beam Induced Depositionxe2x80x9d, J. Vac. Sci., Technol. B 6(1), (1988), 477. However, the exposure time needed is very much greater compared to the resist technique, but remains within justifiable limits per lens.
In addition, by the use of the easily controllable electron beam in the scanning electron microscope, the placement of the exposure field relative to the fiber core and the effective zone of the laser is possible with an up to 100 nm precision, using image processing and scanning microscopy, which is described, for example, by H. W. P. Koops, J. Kretz, and M. Weber in xe2x80x9cCombined Lithographies for the Reduction of Stitching Errors in Lithographyxe2x80x9d, Proc. EIPB94, J. Vac. Sci. Technol. B 12 (6) (1994) 3265-3269. Due to the macrocontrol of the adjustment and the exposure, the exposure process can be automated, controlled by a program.
By the computer control of the exposure and the precalculation of the dose distribution according to measured gradation curves of the resist or deposition process, besides round, elliptical, spherical and hyperbolic lens combinations, lens combinations which are provided with a deviation prism and guide the laser beam so as to orient it can also be jointly realized and precisely adjusted in a refractive surface. The adjustment and the manufacturing process are combined in one, and are superior to conventional methods by at least one order of magnitude. At the same time, the easy controllability and image rotation during the electron-beam exposure represent a method for constructing the lenses which is superior, for example, to laser ablation. These procedures can be easily automated for productive use.